This nonprovisional application claims priority under 35 U.S.C. § 119(a) on Patent Application No(s) . 2002-285157 filed in JAPAN on Sep. 30, 2002, which is(are) herein incorporated by reference.
1. Field of the Invention
The present invention relates to a method of manufacturing a golf ball. More particularly, the present invention relates to an improvement in a method of compressing and molding a cover.
2. Description of the Related Art
A wound ball was once a mainstream of a golf ball. The wound ball comprises a core upon which a rubber thread is wound, and a cover. A compression molding method is employed for forming the cover. In the compression molding method, two half shells formed of a cover material and a core covered with the half shells are put in a mold. The mold is clamped so that the cover material is pressurized and the excessive cover material flows out of a spherical cavity. Air present between the core and the half shell is discharged from the spherical cavity with the flow of the cover material. A part of the air is absorbed in the small spaces between rubber threads.
In recent years, a solid ball has been a mainstream in place of the wound ball. The solid ball comprises a solid core having a single layer or a plurality of layers, and a cover. A compression molding method or an injection molding method can be employed for molding the cover. In general, the injection molding method having excellent mass productivity is employed.
In the injection molding method, first of all, a core is held on the center of a spherical cavity with a holding pin. Next, a molten thermoplastic resin composition is injected into a clearance between a cavity surface and the core. In the final stage of the injection, the holding pin is moved backward. In some cases, therefore, the core is moved from the center with the flow of the resin composition. Due to movement, the thickness of a cover becomes unequal. In some cases, a cover having an unequal thickness is molded due to the deformation of the core which is caused by injection pressure. The unequal thickness of the cover causes the physical properties of the golf ball to be nonuniform. In a golf ball having a small nominal thickness of the cover, particularly, the inequality of the thickness of the cover, seriously affects the physical properties of the golf ball. With respect to the uniformity of the golf ball, the injection molding method has limitations.
If the solid ball is manufactured by the compression molding method, the inequality of the thickness of the cover is caused with difficulty. In the case in which the solid ball is to be formed by the compression molding method, however, a part of the air present between a solid core and a half shell is apt to remain in the cover material. The reason is that the solid core has no space which is seen in a wound core. In the compression molding method, the cover material excessively flows out so that the clearance between a cavity surface and the core is insufficiently filled in some cases. This phenomenon is referred to as bare. In the case in which a cover having a small nominal thickness is to be molded, the air residue and the bare are apt to be generated.